Critical Fracture Load Research Articles

Mechanical properties of a-C:H multilayer films

Multilayered amorphous hydrogenated carbon (a-C:H) films consisting of alternating sublayers with different mechanical properties have been deposited by an electron cyclotron resonance microwave-plasma chemical vapor deposition (ECR MP-CVD) system and modulating substrate bias voltage. The mechanical properties of the multilayer films were determined using nanoindentation and nanoscratch experiments with reference to single a-C:H layers of which the multilayer structure were composed. In nanoindentation tests, the relationship between the film hardness and indentation depth has been obtained over an indentation depth range of 20–500 nm. Since the films tend to fracture under high load in nanoindentation tests, their critical fracture loads were determined. The critical loads for fracturing the multilayered a-C:H films were higher than those of single a-C:H layers. The nanoscratch tests also showed that the multilayered a-C:H films required a higher critical load for scratching fracture. This study implies that the mechanical properties of a-C:H film can be improved by engineering suitable multilayer structures.

MEASUREMENT OF INTERFACIAL FRACTURE TOUGHNESS FOR MICROELECTRONIC PACKAGES

Interfacial adhesion plays an important role in the reliability of plastic encapsulated microelectronic (PEM) devices. Moisture that penetrates through the PEM devices and stays in the weak adhesion location vaporizes when the IC devices are subjected to thermal loading such as solder reflow in the assembly process. Device failures result due to pop-corning and cracking. Reliable PEM devices can be manufactured if there is good adhesion strength between packaging materials. In this paper, measurement and determination of interfacial fracture toughness between electronic materials are addressed. Interfacial fracture toughness between commercially available glob-top material and copper substrate is characterized. Laminated specimens with center pre-crack configuration are prepared and tested using four-point bending (FPB) configuration. The aim is to quantify the critical fracture load during crack initiation. Critical strain energy release rate and its corresponding mode mixity are determined from the measured critical load. Copper/glob-top material (Cu/GT) interface is selected as a case study and it is found that its interfacial fracture energy increases from 4 to 18J/m2 for loading phase angle ranging from 0° (pure mode I) to a mode mixity of 40°. The results will be presented in this paper.

Evaluation of Adhesion Strength for Amorphous SiC Thin Film and DLC/a-SiC Laminated Thin Film Deposited on Steel Substrates for Cutting Tool.

This paper describes the evaluation of adhesion strength between the amorphous Silicon carbide (a-SiC) thin film, Diamond-Like Carbon (DLC)/a-SiC laminated thin film and materials used for the cutting tool. The a-SiC and DLC/a-SiC thin films were deposited on the tungsten carbide steel substrate:K10 and on the high speed steel substrate;SKH51 by Plasma-Enhanced CVD method of the hot cathode PIG discharge type. Scratch tests were carried out for examining the critical fracture load of the films. Finite Element Method (FEM) analyses were also performed to clarify the stress distribution at the interface between the films and substrates. The stress intensity factors KI and KIIand the fracture toughness KC were calculated from results of scratch tests and FEM analyses. The KI, KIIand KC of a-SiC and DLC/a-SiC thin films deposited on the K10 substrate were larger than those of the thin films deposited on the SKH51 substrate. The KI, KII and KC of both of the films were directly proportional to the density of a-SiC thin films. In turning experiments, the delamination area of DLC/a-SiC thin films deposited on the cutting tool of K10 is closely related to the KC obtained from the scratch tests and FEM analyses.

Evaluation of Mode-II Fracture Energy of Adhesive Joints with Different Bond Thickness

Abstract A study on the mode-II edge-sliding fracture behaviour of aluminium-adhesive joints was carried out. Compact pure shear (CPS) adhesive joints of different bond thickness were produced using a rubber-modified epoxy resin as the adhesive. An analytical model was developed to calculate the stress distribution along the bond line of the joint. A crack-closure technique was used to evaluate the mode-II strain energy release rate. G II, as a function of the adhesive bond thickness. The results indicated that for a given applied load, G II increased gradually with the bond thickness. A finite element model (FEM) was also developed to evaluate the stress state along the bond line and the strain energy release rate of the CPS specimens. Consistent results were obtained between the theoretical model and finite element analysis. Scanning electron micrographs of the fracture surface illustrated a mainly interfacial fracture path between the adherends and the adhesive for all adhesive joint specimens. The critical fracture load increased very rapidly with bond thickness in the range 0.02 mm to 0.1 mm but remained constant thereafter. However, the mode-II critical fracture energy rose more gradually as the bond thickness was increased.

Strength of composite sandwich panels containing debonds

A method for determining the critical debond size between the facesheet and the core in composite sandwich panels under in-plane compression is described. The approach uses fracture mechanics together with a buckling criterion for a debonded faceskin. The technique yields predictions for the critical in-plane compressive load for debond propagation as a function of core-to-faceskin debond size, faceskin thickness, lay-up, composite material properties, and honeycomb properties and geometries. A computer program, developed in this work, calculates the critical buckling load and facesheet deformed shape by solving an eigenvalue problem. An experimental study was conducted to determine the onset of delamination buckling in composite sandwich panels containing such flaws. Sandwich panel specimens of graphite/epoxy faceskins and aluminum honeycomb core were constructed with embedded delaminations and with varying faceskin thicknesses and core sizes. Four-point bending tests were conducted such that the faceskin containing the debond was under in-plane compression. The predicted critical fracture loads, computed using the proposed theoretical models which were solved using a numerical computational scheme, closely followed the experimental measurements.

An Analytical and Experimental Study of Mixed-Mode Ductile Fracture under Nonproportional Loading

The paper presents an investigation of mixed-mode ductile fracture for biaxial tension plates containing a central crack of varying inclination angles under non proportional loading. The critical fracture loads at crack initiation as well as crack initia tion angles are obtained for each of the three prescribed nonproportional loading paths. The experimental measurements are then compared to those predicted by finite element analysis based on an anisotropic damage mechanics model which has been developed ear lier by Chow and his co-workers and employed to characterize successfully, various damage-affected fracture problems including mixed-mode crack initiation and propagation under proportional loading [1-5].

Impact Behavior of Coated Low‐Density Bodies

Impact failure of the coating of space shuttle thermal protection tiles was confirmed to occur by a flexure process at low projectile velocities. The critical projectile velocity was measured for a variety of projectile sizes. The same failure process and damage sequence was observed in quasi‐static indentation tests. In these tests, the mechanical behavior agreed reasonably well with the elastic model of an infinite plate bonded to a semiinfinite foundation. The model indicates that the critical fracture load could be increased by increasing coating strength, thickness, or foundation stiffness, or by decreasing the Young's modulus of the coating.

Interaction effects between a crack and a circular inclusion

The plane problem of the interaction between a crack and a circular inclusion is considered. The inclusion is perfectly bonded into an elastic matrix, while the crack is arbitrarily oriented with respect to the inclusion. Only tensile loads perpendicular to the axis of the crack and acting on the matrix are considered. The basic problem of fracture initiation from the ends of the crack is thoroughly studied. The relevant fracture characteristic quantities incorporating the critical fracture load and the angle of crack extension are determined. The usual cases of a hole and a metallic inclusion embedded into an epoxy matrix are considered. Useful results concerning the dependence of the above two fracture characteristic quantities of the crack extension on the more vulnerable crack tip, the relative location of the crack and the inclusion and the material properties of the composite plate are derived.

A study of failure in the post yield regime using single edge-notched tension specimens

Three post yield analyses have been performed on fracture data obtained from single edge-notched tension specimens of widths 25, 15 and 8 mm and constant thickness 25 mm made of a 1% Cr MoV high temperature bolting steel. The three methods were: the compliance technique, and two curve fitting procedures which utilize solutions to the Bilby, Cottrell and Swinden-Dugdale model and are based upon the load/gauge length displacement and the load/external crack opening displacement. All three methods gave results in good agreement with each other. The fracture toughness values determined increased with decreasing crack length, the toughness for the shortest crack length (2.5 mm) was twice that for the longest (10.5 mm). This is associated with an observed increase in ductility on the fractured surfaces which were still predominantly cleavage in appearance. This rise in toughness value occurs for crack sizes from 6 to 8 times the characteristic length {ie233-1}. In ferritic steels the results imply that post yield failure analyses which use valid toughness values for assessing the integrity of plant containing shallow cracks may underestimate critical defect sizes and fracture loads. Hence safe results will be obtained provided failure is predominantly by a cleavage mechanism. There is also the possibility that toughness values obtained from data on invalid specimens may be greater than the fracture toughness values obtained from valid tests at the same temperature.

Incipient fracture angle, fracture loci and critical stress for mixed mode loading

A prediction of the direction of incipient crack growth in brittle-like materials and the associated fracture loci under mixed mode loading is proposed. It is postulated that the direction of unstable crack propagation is determined by the “weakest” near-tip element defined as the one which would relax maximum potential energy upon prospective crack extension. Starting from the energy rate principle of crack extension (Eshelby energy-momentum tensor and Rice J- internal vector) it is deduced that a crack will extent in the direction along which the following stress criterion is satisfied, (δ θθ 2 − δ rr 2) → maximum (for δ θθ > 0) The fracture angle in pure Mode II (70.4° away from the original straight path) is shown to be unstable in the sense that any slight tension along the crack (non-singular at the crack tip) affects considerably (up to 22%) the directionality of crack extension. It appears to be sensitive to the extent of the near-tip zone ( r 0 ) in which linear elasticity does not hold and the non-singular stress term (squared). The fracture loci in mixed mode loading (generated by projecting the J- integral vector along the prospective fracture path and letting this scalar function attain a critical value) is quadratic in K 1 and K 2 with an interactive cross product term K 1 × K 2 . The suggested criterion with its implication in predicting critical fracture load, exhibits behavior which is consistent with experimental observations collected from several sources. The common and uncommon features with respect to other known criteria are compared and discussed.

The surface cracking of glassy polymers under a sliding spherical indenter

Glassy polymers crack under a sliding hard spherical indenter in a way that is observed in other brittle materials. A series of curved cracks concave to the wake of the indenter are formed and these can penetrate to a depth of a few hundredths of a millimetre. With polystyrene the additional stress imposed by sliding reduces the critical load to fracture under normal loading conditions by 95% and the presence of an active environment, methanol, reduced the critical fracture load for sliding by 83%. Other glassy polymers which are generally considered tough, cracked in the presence of agents which could promote environmental stress cracking. The stress necessary to cause cracking and the crack size obeyed laws pertaining to fracture under elastic stress conditions. Although the flaws observed are referred to as cracks it is recognized that they could possibly be crazes. The introduction of surface damage in the form of cracks or crazes is important practically, since the inadvertent scratching or abrasion of the polymer surfaces may introduce flaws which under normally acceptable stress-environmental conditions could lead to failure. It is also noted that the flaws are detrimental to the polymer's optical properties.

ChemInform Abstract: HERTZIAN FRACTURE OF VITREOUS CARBON

AbstractDas Bruchverhalten von glasigem Kohlenstoff bei Aufprall von Stahlkugel‐ Prüfkörpern wird mit den Theorien von Hertz verglichen.

Hertzian Fracture of Vitreous Carbon

Hertzian fracture produced by steel ball indenters on vitreous carbon is qualitatively similar to Hertzian fracture of soda‐lime glass. The bulk material obeys Auerbach's law, i.e. the critical fracture load is proportional to the ball radius. However, a surface layer of material, which is denser than the bulk, is stronger and does not obey Auerbach's law. The fracture energy calculated from Auerbach's law and the theory of Frank and Lawn is more than an order of magnitude lower than that measured by conventional methods. Part of this discrepancy arises from the fact that the ring crack is 17% larger than the circle of contact. The fracture surface energy of vitreous carbon as determined by a somewhat empirical application of Auerbach's law is ∼3.5 J/m2. The Hertzian fracture stress of vitreous carbon, like that of soda‐lime glass, approaches the transverse rupture strength (TRS) when large indenters are used.

Section-and-Etch Study of Hertzian Fracture Mechanics

A section-and-etch technique is used to study the mechanics of Hertzian cone-crack growth in glass. With a suitable choice of test environment the cone cracks propagate at a convenient rate through successive phases of stability at constant indenter load. Systematic measurements of the etched-crack lengths as a function of indentation time permit a detailed description of the fracture mechanics. Within a certain range of indenter load, the cone crack is observed to grow as a shallow surface ring to a critical depth prior to full development. This directly confirms a salient feature of the energy balance theory of Hertzian fracture outlined in earlier papers. The current status of the long-standing Auerbach law, which relates the critical fracture load linearly to the indenter radius, is discussed in the light of the present evidence.